Crystal Structures of a Cubic Tin(II) Germanate, α-Sn6GeO8, and a Tetragonal Tin(II) Silicate, γ-Sn6SiO8

A cubic tin(II) germanate, α-Sn6GeO8 (space group F4̅3m, a = 10.52521(2) Å, and Z = 4), has been synthesized by both regular hydrothermal and microwave-assisted hydrothermal methods, and the crystal structure of this material has been solved by Rietveld refinement of synchrotron powder X-ray diffraction (PXRD) data. The crystal structure is analogous to α-Sn6SiO8 and is therefore related to the zinc blende structure comprising a face-centered cubic array of [Sn6O8]4– anionic clusters with Ge4+ cations occupying half of the tetrahedral holes. Variable-temperature PXRD has revealed that tin(II) germanate has high thermal stability: remaining stable at 950 K and mostly decomposing over the range 984–1034 K. The tin(II) germanate has been further characterized by X-ray fluorescence (XRF), Raman, and diffuse reflectance (DR) UV–vis spectroscopies. In addition, variable-temperature PXRD studies have revealed the formation of a tetragonal tin(II) silicate polymorph, γ-Sn6SiO8 (space group I4̅, a = 7.30414(6) Å, c = 10.53731(6) Å, and Z = 2), at temperatures below 170 K. The crystal structure of γ-Sn6SiO8 has been elucidated by Rietveld refinement. While a transition to a tetragonal polymorph is observed upon cooling α-Sn6SiO8, no corresponding transition is observed for α-Sn6GeO8, which retains its cubic structure over the probed temperature range.

* daniel.parsons@diamond.ac.uk Contents Discussion on the 5.00 SnCl2: 0.83 SiO2: x NaOH: 676 H2O gel system S-2 XRF spectrum of α-Sn6GeO8 S-4 Diffuse reflectance (DR) UV-Vis spectra and interpretation S-5 VT-PXRD patterns showing the formation of β-Sn6GeO8 S-7 Rietveld refinement of α-Sn6GeO8 at 100 K S-8 Thermal expansion of α-Sn6GeO8 S-8 Space group selection for γ-Sn6SiO8 S-10 S-2 Discussion on the 5.00 SnCl2: 0.83 SiO2: x NaOH: 676 H2O gel system Tin(II) chloride was substituted for tin(II) oxalate in reactive gels with varying base content, as the hydrolysis of tin(II) chloride can increase the acidity of the solution. The gels employed had the following composition (in millimoles): 5.00 SnCl2: 0.83 SiO2: x NaOH: 676 H2O. Following 30 minutes of stirring, the homogenized gels were heated at 160 °C in a convection oven for 10 hours in Teflon-lined autoclaves then recovered by vacuum filtration.
The products formed from synthetic attempts employing tin(II) chloride varied depending on the base content of the gel, as shown in Table S1, which lists the product phases observed as the base content in the gel is increased. If it is assumed that the hydrolysis of one equivalent of tin(II) chloride proceeds fully to yield two equivalents of aqueous protons, correlations between the observed products and expected gel pH may be made. In gels which were expected to be acidic, i.e. an insufficient amount of sodium hydroxide was added to neutralize the acidity of the fully hydrolysed tin(II) chloride, the products were pale yellow in color and their PXRD patterns contain reflections corresponding to abhurite (Fig. S1). Abhurite is a tin(II) oxide hydroxychloride with idealized formula: Sn21O6Cl16(OH)14, that may be produced synthetically, but has also been observed as a corrosion product on the surface of tin and pewter artifacts that have spent long periods submerged in an aqueous environment. 24 In gels with a base content equal to the concentration required to exactly neutralize all protons produced by the complete hydrolysis of tin(II) chloride ([OH -]/HCl = 1.00), α-SnO is the only phase present in the product PXRD pattern ( S-3 Figure S1. PXRD patterns of the products of hydrothermal synthesis in the 5.00 SnCl2: 0.83 SiO2: x NaOH: 676 H2O gel system for x = 5.00 and x = 7.50. Figure S2. PXRD pattern of the product of a hydrothermal synthesis in the 5.00 SnCl2: 0.83 SiO2: x NaOH: 676 H2O gel system for x = 10.00. Differences in relative intensity between the reference SnO pattern (PDF 04-005-4540) and the experimental PXRD pattern may be attributed to preferred orientation in the latter. Greater intensity than would be expected for 00l reflections is frequently encountered in PXRD patterns of SnO, owing to the platy nature of the crystallites, a consequence of the layered crystal structure. 1 S-4 Figure S3. PXRD patterns of the products of hydrothermal synthesis in the 5.00 SnCl2: 0.83 SiO2: x NaOH: 676 H2O gel system for x = 12.50 and x = 15.00.
XRF spectrum of α-Sn6GeO8 Figure S4. XRF spectrum of α-Sn6GeO8 recorded on a HORIBA Jobin Yvon XGT-7000V X-Ray Analytical Microscope. The energies on the x-axis are in keV. The Ge and Sn peaks are labelled in the figure. The unlabelled peaks in the spectrum correspond to peaks for Rh, from the X-ray source in the instrument, and Si, from the glass slide on which the sample pellet was mounted.

S-5
Diffuse reflectance (DR) UV-Vis spectra and interpretation Figure S5. Diffuse reflectance UV-Vis spectrum of α-Sn6GeO8. The discontinuity at ca. 320 nm is an instrumental artifact owing to the changeover from the deuterium source to the tungsten source in the instrument at this wavelength. where F(hν) and R(hν) are the Kubelka-Munk function and the reflectance, respectively, at given energy hν. The Tauc plot is obtained by plotting F(hν) 2 as a function of hν.
Eq. S1. Thermal expansion of α-Sn6GeO8 Figure S9. A plot of lattice constant as a function of temperature for α-Sn6GeO8. The equation for the line of best fit and the R 2 value for the fit are included on the plot. The lattice constants at 100 K and 290 K were determined by Rietveld refinements of 30-minute synchrotron PXRD data collections. The lattice constants at 451 K, 628 K, 784 K and 950 K were determined by Pawley fits, performed in GSAS-II, for 10-minute synchrotron PXRD data collections.
An indexing procedure performed in GSAS-II on a γ-Sn6SiO8 PXRD pattern, recorded at 100 K, revealed the highest figure of merit for a body-centred tetragonal cell belonging to the I---extinction class.
The critical difference between the 3 candidate space groups, I4 ̅ (82), I4 ̅ 2m (119) and I4 ̅ m2 (121), is the presence of a mirror plane and rotation axis in the latter two, whereas these symmetry elements are absent in I4 ̅ . Mirror planes are present in α-Sn6SiO8, which occur co-incident with the {110} lattice planes. 3-rotation axes are also present in α-Sn6SiO8, which run in the ⟨111⟩ directions, such that each rotation axis in the structure runs parallel to a mirror plane, as highlighted in Figure S12. The highest order rotation axis in the space groups I4 ̅ 2m and I4 ̅ m2 is a 2-rotation axis, therefore the 3-rotation axis is not preserved in the phase transition from the cubic cell to the tetragonal. Any structural change which eliminates the 3-rotation axis is also likely to eliminate the mirror planes which run parallel to them; therefore I4 ̅ appeared the most plausible space group for γ-Sn6SiO8 and was chosen as the space group with which to construct an initial model to be used in a Rietveld refinement. Construction of the model was aided by using maximal subgroup-minimal supergroup relationships, as F4 ̅ 3m is a minimal supergroup of I4 ̅ m2, which is in turn a minimal supergroup of I4 ̅ . Accordingly, the splitting of Wyckoff positions through maximal subgroups guided construction of the tetragonal a b c S-10 model, in addition to considerations of the likely spatial relationship between the tetragonal cell and the cubic predecessor.
A model may also be constructed in the I4 ̅ m2 space group using the maximal subgroup relationship with F4 ̅ 3m; however, refining such a model against the experimental pattern leads to poor fits (Rwp = 16.35 %). Moreover, a refined model set in I4 ̅ m2 provides no calculated intensity for several observed reflections, including the (211) and (411) reflections. Ultimately, this further supports that I4 ̅ is the true space group. The critical difference between models constructed in I4 ̅ m2 and I4 ̅ are the site symmetries in the former and the way this impacts the structure. In models set in the I4 ̅ space group, the Sn2, O1 and O2 sites occupy the 8g general position, but in an I4 ̅ m2 model, these atoms must reside on special positions: with Sn2 residing on the 8g position (with ..2 point symmetry) and O1 and O2 both residing on the 8i position (.m. point symmetry). Accordingly, the point symmetries of the special positions impose spatial limitations on where these atoms may reside in the refined I4 ̅ m2 model. For the Sn2 site which requires atoms to be located on (x,x,0) positions, all four Sn2 atoms in each cluster must reside within the same plane at a constant z value which prevents the distortions in the cluster that are observed in the refined I4 ̅ model (Rwp = 9.30 %) from occurring in the I4 ̅ m2 model (Rwp = 16.35 %).